Surfactant

ABSTRACT

A surfactant has the formula (R 1 —X) n Z, where R 1  is an aliphatic group comprising a C 10 -C 25  principal straight chain bonded at a terminal carbon atom thereof to X, and comprising at least one C 1 -C 6  side chain. X is a charbed head group, Z is a counterion, and n is an integer which ensures that the surfactant is charge neutral.

FIELD OF THE INVENTION

[0001] The present invention relates to a surfactant, and in particularto a surfactant thickening agent for use in hydrocarbon recovery.

BACKGROUND OF THE INVENTION

[0002] In the recovery of hydrocarbons, such as oil and gas, fromnatural hydrocarbon reservoirs, extensive use is made of wellbore fluidssuch as drilling fluids, completion fluids, work over fluids, packerfluids, fracturing fluids, conformance or permeability control fluidsand the like.

[0003] In many cases significant components of wellbore fluids arethickening agents, usually based on polymers or viscoelasticsurfactants, which serve to control the viscosity of the fluids. Typicalviscoelastic surfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and potassium oleate, solutions of which form gelswhen mixed with corresponding activators such as sodium salicylate andpotassium chloride.

[0004] The surfactant molecules are characterized by having one longhydrocarbon chain per surfactant headgroup. In the viscoelastic gelledstate these molecules aggregate into worm-like micelles. Gel breakdownoccurs rapidly when the fluid contacts hydrocarbons which cause themicelles to change structure or disband.

[0005] In practical terms the surfactants act as reversible thickeningagents so that, on placement in subterranean reservoir formations, theviscosity of a wellbore fluid containing such a surfactant variessignificantly between water- or hydrocarbon-bearing zones of theformations. In this way the fluid is able preferentially to penetratehydrocarbon-bearing zones.

[0006] The use of viscoelastic surfactants for fracturing subterraneanformations is discussed in EP-A-0835983.

[0007] A problem associated with the use of viscoelastic surfactants isthat stable oil-in-water emulsions are often formed between the lowviscosity surfactant solution (i.e. broken gel) and the reservoirhydrocarbons. As a consequence, a clean separation of the two phases canbe difficult to achieve, complicating clean up of wellbore fluids. Suchemulsions are believed to form because conventional wellbore fluidviscoelastic surfactants have little or no solubility in organicsolvents.

[0008] A few anionic surfactants exhibit high solubility in hydrocarbonsbut low solubility in aqueous solutions. A well known example is sodiumbis(2-ethylhexyl) sulphosuccinate, commonly termed aerosol OT or AOT(see K. M. Manoj et al., Langmuir, 12, 4068-4072, (1996)). However, AOTdoes not form viscoelastic solutions in aqueous media, e.g. the additionof salt causes precipitation.

[0009] A number of cationic surfactants, based on quaternary ammoniumand phosphonium salts, are known to exhibit solubility in water andhydrocarbons and as such are frequently used as phase-transfer catalysts(see C. M. Starks et al., Phase-Transfer Catalysis, pp. 125-153, Chapmanand Hall, New York (1994)). However, those cationic surfactants whichform viscoelastic solutions in aqueous media are poorly soluble inhydrocarbons, and are characterized by values of K_(ow) very close tozero, K_(ow) being the partition coefficient for a surfactant in oil andwater (K_(ow)=C_(o)/C_(w), where C_(o) and C_(w) are respectively thesurfactant concentrations in oil and water). K_(ow) may be determined byvarious analytical techniques, see e.g. M. A. Sharaf, D. L. Illman andB. R. Kowalski, Chemometrics, Wiley Interscience, (1986), ISBN0471-83106-9.

[0010] Typically, high solubility of the cationic surfactant inhydrocarbon solvents is promoted by multiple long-chain alkyl groupsattached to the head group, as found e.g. inhexadecyltributylphosphonium and trioctylmethylammonium ions. Incontrast, cationic surfactants which form viscoelastic solutionsgenerally have only one long unbranched hydrocarbon chain per surfactantheadgroup.

[0011] The conflict between the structural requirements for achievingsolubility in hydrocarbons and for the formation of viscoelasticsolutions generally results in only one of these properties beingachieved.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a surfactantwhich is suitable for reversibly thickening water-based wellbore fluidsand is also soluble in both organic and aqueous fluids.

[0013] A first aspect of the present invention provides a surfactanthaving the formula (R₁—X)_(n)Z. R₁ is an aliphatic group comprising aprincipal straight chain bonded at a terminal carbon atom thereof to X,the straight chain having a length such that a viscoelastic gel isformable by the surfactant in aqueous media; and further comprising atleast one side chain (the carbon atoms of the side chain not beingcounted with the carbon atoms of the principal straight chain) which isshorter than said principal straight chain, said side chain enhancingthe solubility of the surfactant in hydrocarbons, and being sufficientlyclose to said head group and sufficiently short such that the surfactantforms micelles in said viscoelastic gel. X is a charged head group, Z isa counterion, and n is an integer which ensures that the surfactant ischarge neutral. Preferably the principal straight chain is a C₁₀-C₂₅straight chain. Preferably the side chain is a C₁-C₆ side chain.

[0014] X may be a carboxylate (—COO⁻), quaternary ammonium (—NR₂R₃R₄ ⁺),sulphate (—OSO₃ ⁻), or sulphonate (—SO₃ ⁻) charged group; N being anitrogen atom, and R₂, R₃ and R₄ being C₁-C₆ aliphatic groups, or one ofR₂, R₃ and R₄ being a C₁-C₆ aliphatic group and the others of R₂, R₃ andR₄ forming a five- or six-member heterocylic ring with the nitrogenatom.

[0015] When X is a carboxylate, sulphate, or sulphonate group, Z may bean alkali metal cation (in which case n is one) or an alkaline earthmetal cation (in which case n is two). Preferably Z is Na⁺ or K⁺.

[0016] When X is a quaternary ammonium group, Z may be a halide anion,such as Cl⁻ or Br⁻, or a small organic anion, such as a salicylate. Inboth these cases n is one. Preferably the principal straight chain is aC₁₆-C₂₄ chain. More preferably it is a C₁₈ or a C₂₂ chain.

[0017] We have found that surfactants of this type are suitable for useas wellbore thickening agents, being soluble in both water andhydrocarbon-based solvents but retaining the ability to form aqueousviscoelastic solutions via micellar aggregation. This combination ofproperties is believed to be caused by the branching off from theprincipal straight chain of the C₁-C₆ side chain. The side chainapparently improves the solubility in hydrocarbon solvents by increasingthe hydrophobicity of the R₁ aliphatic group.

[0018] By “viscoelastic”, we mean that the elastic (or storage) modulusG′ of the fluid is greater than the loss modulus G″ as measured using anoscillatory shear rheometer (such as a Bohlin CVO 50) at a frequency of1 Hz. The measurement of these moduli is described in An Introduction toRheology, by H. A. Barnes, J. F. Hutton, and K. Walters, Elsevier,Amsterdam (1997).

[0019] In use, the enhanced solubility of the surfactant inhydrocarbon-based solvents can reduce the tendency for an emulsion toform between reservoir hydrocarbons and a broken surfactant gel based onthe surfactant. It may also inhibit the formation of emulsions bynatural surfactants in crude oil, such as naphthenic acids andasphaltenes. Additionally, dissolution of at least some of thesurfactant molecules into the reservoir hydrocarbons can speed upbreakdown of the gel.

[0020] Preferably, the side chain is a C₁-C₃ chain. We have found that,surprisingly, the solubility of the surfactant in hydrocarbon tends toincrease as the size of the side chain decreases. We believe this isbecause smaller side chains cause less disruption to the formation ofinverse micelles by the surfactant in the hydrocarbon, such inversemicelles promoting solubility in the hydrocarbon.

[0021] By altering the degree and type of branching from the principalstraight chain, the surfactant can be tailored to be more or lesssoluble in a particular hydrocarbon. However, preferably the side chainis bonded to said terminal (α), neighbouring (β) or next-neighbouring(γ) carbon atom of the principal chain. More preferably it is bonded tothe α carbon atom. We believe that locating the side chain close to thecharged head group promotes the most favourable combinations ofviscoelastic and solute properties.

[0022] Preferably the side chain is a methyl or ethyl group. There maybe two side groups, e.g. a methyl and an ethyl group bonded to the αcarbon atom.

[0023] The principal straight chain may be unsaturated.

[0024] Preferably the surfactant is an alkali metal salt of 2-methyloleic acid or 2-ethyl oleic acid.

[0025] A second aspect of the invention provides a viscoelasticsurfactant having a partition coefficient, K_(ow), of at least 0.05,K_(ow) being measured at room temperature with respect to heptane andwater. More desirably K_(ow) is in the range from 0.05 to 1 and mostdesirably it is in the range 0.05 to 0.5. The surfactant may be asurfactant of the first aspect of the invention.

[0026] A third aspect of the invention provides an acid surfactantprecursor to the surfactant of the first aspect of the invention, theacid surfactant precursor having the formula R₁—Y. R₁ is an aliphaticgroup comprising a C₁₀-C₂₅ principal straight chain bonded at a terminalcarbon atom thereof to Y, and comprising at least one C₁-C₆ side chain.Y is a carboxylate (—COOH), sulphate (—OSO₃H), or sulphonate (—SO₃H)group.

[0027] In solution, acid surfactant precursors can be converted to thesalt form, e.g. by neutralisation with the appropriate alkali or by theaddition of the appropriate salt, to form surfactants of the firstaspect of the invention.

[0028] A fourth aspect of the present invention provides a wellborefluid comprising:

[0029] (a) water,

[0030] (b) a thickening amount of the surfactant of the first or secondaspect of the invention, and

[0031] (c) an effective amount of a water-soluble, inorganic saltthickening activator.

[0032] Preferably the thickening activator is an alkali metal salt, suchas KCl.

[0033] The surfactant is typically present in the fluid in aconcentration of from 0.5 to 10 wt % (and more typically 0.5 to 5 wt %)and the thickening activator is typically present in the fluid in aconcentration of from 1 to 10 wt %.

[0034] Desirably the wellbore fluid has a gel strength in the range 3 to5 at room temperature, the gel strength falling to a value of 1 oncontact with hydrocarbons such as heptane.

[0035] Desirably the wellbore fluid has a viscosity in the range 20 to1000 (preferably 100 to 1000) centipoise in the shear rate range 0.1-100(preferably 0.1-1000) s⁻¹ at 60° C., the viscosity falling to a value inthe range 1 to 200 (preferably 1 to 50) centipoise on contact withhydrocarbons such as heptane, the viscosity being measured in accordancewith German DIN standard 53019.

[0036] A fifth aspect of the present invention provides for use of thewellbore fluid of the fourth aspect of the invention as a fracturingfluid, a lubricant or an emulsion breaker.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Specific embodiments of the present invention will now bedescribed with reference to the following drawings in which:

[0038]FIG. 1 shows schematically steps in the synthesis of an α-branchedfatty acid metal salt,

[0039]FIG. 2 shows schematically steps in the synthesis of a β-branchedfatty acid metal salt,

[0040]FIG. 3 shows schematically steps in the synthesis of a γ-branchedfatty acid metal salt,

[0041]FIG. 4 shows a graph of gel strength against time for potassiumoleate gel and potassium 2-methyl oleate gel, and

[0042]FIG. 5 shows gel strength codings.

DETAILED DESCRIPTION

[0043] Synthetic routes to α-, β- and γ-branched derivatives of variousfatty acids are shown schematically in FIGS. 1 to 3. The type of fattyacid and length of side chain, R, can be varied. If desired, two sidechains can be attached to the same fatty acid carbon atom.

[0044] A first step in a preparation of an α-branched derivative of aC₁₀-C₂₅ straight chain acid is the formation of an α-branch on themethyl ester of the acid. The α-branched ester can then be saponifiedwith metal hydroxide to generate the acid salt (and thence the acid, ifrequired).

[0045] The following example describes in more detail the preparationand characterisation of 2-methyl oleic acid.

EXAMPLE

[0046] 1. Preparation of 2-methyl Methyl Oleate

[0047] Sodium hydride (60% dispersion, 8 g, 0.2 mol) was washed withheptane (2×15 ml) and then suspended in tetrahydrofuran (THF) (300 ml).1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) (26 g, 0.2mol) was added and the mixture was stirred under an atmosphere ofnitrogen. Methyl oleate (67.46 ml, 0.2 mol) was added dropwise over aperiod of two hours and the resulting mixture was heated to reflux for12 hours and then cooled to 0° C. Methyl iodide (0.2 mol) was then addeddropwise and the reaction mixture was again heated to reflux for afurther two hours. Next the reaction mixture was cooled to 0° C. andquenched with water (15 ml), concentrated in vacuo and purified bycolumn chromatography (SiO₂, 1:9, diethyl ether: petroleum ether) togive 2-methyl methyl oleate as a yellow oil (50 g, 0.16 mol, 81%).

[0048] 2. Preparation of 2-methyl Oleic Acid

[0049] The 2-methyl methyl oleate from the above reaction (40 g, 0.13mol) was dissolved in a (3:2:1) methanol, THF and water mixture (300ml), and potassium hydroxide (14.4 g, 0.26 mol) was added and thereaction heated to reflux for 15 hours. The reaction mixture was thencooled and neutralised using dilute hydrochloric acid. The organic layerwas separated and concentrated in vacuo, and was then purified by columnchromatography (SiO₂, (2:8) ethyl acetate: petroleum ether) to give2-methyl oleic acid as an oil.

[0050] 3. Characterisation

[0051] A rigid gel was formed when a 10% solution of potassium 2-methyloleate (the potassium salt of the 2-methyl oleic acid prepared above)was mixed with an equal volume of a brine containing 16% KCl.

[0052] Contacting this gel with a representative hydrocarbon, such asheptane, resulted in a dramatic loss of viscosity and the formation oftwo low viscosity clear solutions: an upper oil phase and a loweraqueous phase. The formation of an emulsion was not observed. Thin-layerchromatography and infrared spectroscopy showed the presence of thebranched oleate in both phases.

[0053] The gel is apparently broken by a combination of micellarrearrangement and dissolution of the branched oleate in the oil phase.Consequently the breaking rate of the branched oleate is faster thanthat of the equivalent unbranched oleate. This is demonstrated in FIG. 4which is a graph of gel strength against time at room temperature for apotassium oleate (unbranched) gel and the potassium 2-methyl oleate(branched) gel. Both gels were prepared from 10% solutions of therespective oleate mixed with equal volumes of a brine containing 16%KCl. Each gel was then contacted with an equal volume of heptane.

[0054] Gel strength is a semi-quantitative measure of the flowability ofsurfactant-based gel relative to the flowability of the precursor fluidbefore addition of the surfactant. There are five gel strength codingsranging from 1 (flowability of the original precursor fluid) to 5(deformable, non-flowing gel). A particular gel is given a coding bymatching the gel to one of the illustrations shown in FIG. 5.

[0055] Using infra-red spectroscopy, the value of K_(ow) for thepotassium 2-methyl oleate of the broken branched gel was measured as0.11. In contrast the value of K_(ow) for the potassium oleate of thebroken unbranched gel was measured as effectively zero.

[0056] The rapid breakdown of the branched oleate surfactant gels, withlittle or no subsequent emulsion, leads to the expectation that thesegels will be particularly suitable for use as wellbore fluids, such asfluids for hydraulic fracturing of oil-bearing zones. Excellent clean upof the fluids and reduced impairment of zone matrix permeability canalso be expected because emulsion formation can be avoided.

[0057] While the invention has been described in conjunction with theexemplary embodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1. A surfactant having the formula (R₁—X)_(n)Z, where R₁ is an aliphaticgroup comprising a C₁₀-C₂₅ principal straight chain bonded at a terminalcarbon atom thereof to X, and comprising at least one C₁-C₆ side chain;X being a charged head group, Z being a counterion, and n being aninteger which ensures that the surfactant is charge neutral.
 2. Asurfactant according to claim 1, wherein X is a carboxylate (—COO⁻),quaternary ammonium (—NR₂R₃R₄ ⁺), sulphate (—OSO₃ ⁻), or sulphonate(—SO₃ ⁻) charged group; N being a nitrogen atom, and R₂, R₃ and R₄ beingC₁-C₆ aliphatic groups, or one of R₂, R₃ and R₄ being a C₁-C₆ aliphaticgroup and the others of R₂, R₃ and R₄ forming a five- or six-memberheterocylic ring with the nitrogen atom.
 3. A surfactant according toclaim 1 or 2, wherein said side chain is a C₁-C₃ chain.
 4. A surfactantaccording to any one claims 1 to 3, wherein said principal straightchain is a C₁₆-C₂₄ chain.
 5. A surfactant according to any one of claims1 to 4, wherein said side chain is bonded to said terminal carbon atom.6. A surfactant according to any one of claims 1 to 5, wherein said sidechain is a methyl or ethyl group.
 7. A surfactant according to any oneof claims 1 to 6, wherein said principal straight chain is unsaturated.8. A surfactant according to any one of claims 1 to 7, which is analkali metal salt of 2-methyl oleic acid or 2-ethyl oleic acid.
 9. Asurfactant according to any one of claims 1 to 8, having a partitioncoefficient, K_(ow), of at least 0.05.
 10. A viscoelastic surfactanthaving a partition coefficient, K_(ow), of at least 0.05.
 11. Aviscoelastic surfactant according to claim 10, wherein K_(ow) is in therange 0.05 to
 1. 12. Use of the surfactant of one of claims 1 to 11 toform a viscoelastic gel.
 13. An acid surfactant precursor to thesurfactant of one of claims 1 to 9, the acid surfactant precursor havingthe formula R₁—Y, where R₁ is an aliphatic group comprising a C₁₀-C₂₅principal straight chain bonded at a terminal carbon atom thereof to Y,and comprising at least one C₁-C₆ side chain, Y being a carboxylate(—COOH), sulphate (—OSO₃H), or sulphonate (—SO₃H) group.
 14. A wellborefluid comprising: (a) water, (b) a thickening amount of the surfactantof one of claims 1 to 11, and (c) an effective amount of awater-soluble, inorganic salt thickening activator.
 15. A wellbore fluidaccording to claim 14, wherein the thickening activator is an alkalimetal salt.
 16. A wellbore fluid according to claim 14 or 15, whereinthe surfactant is present in the fluid in a concentration of from 0.5 to10 wt % and the thickening activator is present in the fluid in aconcentration of from 1 to 10 wt %.
 17. A wellbore fluid according toany one of claims 14 to 16, having a gel strength in the range 3 to 5 atroom temperature, the gel strength falling to a value of 1 on contactwith heptane.
 18. Use of the wellbore fluid of one of claims 14 to 17 asa fracturing fluid, a lubricant or an emulsion breaker.
 19. A surfactanthaving the formula (R₁—X)_(n)Z, where X is a charged head group; R₁ isan aliphatic group comprising a principal straight chain bonded at aterminal carbon atom thereof to X, the straight chain having a lengthsuch that a viscoelastic gel is formable by the surfactant in aqueousmedia; R₁ further comprises at least one side chain which is shorterthan said principal straight chain, said side chain enhancing thesolubility of the surfactant in hydrocarbons, and being sufficientlyclose to said head group and sufficiently short such that the surfactantforms micelles in said viscoelastic gel; Z is a counterion; and n is aninteger which ensures that the surfactant is charge neutral.